1. Field of the Invention
The present invention relates to a disk array apparatus, and in particular to a management technique for a differential LU in a snapshot.
2. Description of the Related Art
In a computer system, a copy of a storage volume storing original data may be generated, for example, in the case in which data to be stored in a storage device is backed up. In this case, the storage volume storing original data and a storage volume storing copy data are required to have the same contents and keep consistency. Thus, it is necessary to stop update of the storage volume storing original data until generation of the copy is completed. However, in a computer system or the like in which high-availability is required, access to the storage volume storing original data cannot be stopped in some cases.
Therefore, a technique called snapshot has been developed. The snapshot keeps consistency at a certain point in time between the storage volume storing original data and the storage volume storing copy data such that the original data at a point in time can be referred to even if the original data is updated after the point in time (See JP-A-2001-306407).
According to the technique of snapshot, in the case in which the original data is updated after the point when consistency should be kept, the data at the point when consistency should be kept is stored in another storage volume. In other words, the original data is kept as it is unless it is updated, and the data at the point when consistency should be kept is stored in another storage volume as differential data if it is updated.
Although a snapshot function has an advantage that a capacity of a differential LU can be reduced compared with a capacity of a primary LU, differential data increases if an amount of data to be written from a host increases. Then, when utilization of the differential LU has reached 100%, the snapshot cannot be maintained. Consequently, if a large number of LUs are set as differential LUs, the LUs cannot be used for other applications in the case in which a large amount of data is not written.